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137 lines
4.8 KiB
Markdown
137 lines
4.8 KiB
Markdown
# Modifying values
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Once a JSON value exists, its content can be changed: elements can be added, replaced, merged, and removed. This page
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gives an overview of the available operations. For read access, see [element access](element_access/index.md).
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## Adding to arrays
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New elements are appended to an array with [`push_back`](../api/basic_json/push_back.md) or constructed in place with
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[`emplace_back`](../api/basic_json/emplace_back.md). If the value is `#!json null`, it is converted to an array first, so
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these functions can also be used to build an array from scratch.
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```cpp
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json j; // null
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j.push_back(1); // [1]
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j.push_back(2); // [1,2]
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j.emplace_back(3); // [1,2,3]
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// operator+= is a shorthand for push_back
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j += 4; // [1,2,3,4]
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```
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## Adding to objects
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The most common way to add or replace a member is [`operator[]`](element_access/unchecked_access.md), which inserts the
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key if it does not exist yet:
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```cpp
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json j;
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j["name"] = "Mary"; // {"name":"Mary"}
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j["name"] = "John"; // {"name":"John"} (replaced)
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```
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[`emplace`](../api/basic_json/emplace.md) inserts a member only if the key is not already present, and reports whether
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the insertion happened — useful for "add if absent" semantics.
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## Merging objects
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To merge one object into another, [`update`](../api/basic_json/update.md) copies all members from another object
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(similar to Python's `dict.update`). This is the idiomatic way to combine two objects. It has two modes:
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- By default, the merge is shallow: existing keys are overwritten, even if both values are objects.
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- With `merge_objects = #!cpp true`, keys whose values are objects in both JSON values are merged recursively.
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Everything else is overwritten. In particular, arrays are replaced, not concatenated.
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??? example
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```cpp
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#include <iostream>
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#include <iomanip>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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using namespace nlohmann::literals;
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int main()
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{
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// create two JSON objects
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json o1 = R"( {"color": "red", "price": 17.99, "names": {"de": "Flugzeug"}} )"_json;
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json o2 = R"( {"color": "blue", "speed": 100, "names": {"en": "plane"}} )"_json;
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json o3 = o1;
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// add all keys from o2 to o1 (updating "color", replacing "names")
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o1.update(o2);
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// add all keys from o2 to o1 (updating "color", merging "names")
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o3.update(o2, true);
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// output updated object o1 and o3
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std::cout << std::setw(2) << o1 << '\n';
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std::cout << std::setw(2) << o3 << '\n';
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}
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```
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Output:
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```json
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{
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"color": "blue",
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"names": {
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"en": "plane"
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},
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"price": 17.99,
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"speed": 100
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}
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{
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"color": "blue",
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"names": {
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"de": "Flugzeug",
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"en": "plane"
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},
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"price": 17.99,
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"speed": 100
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}
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```
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A common use of the recursive mode is combining defaults with user settings. Nested defaults that the user did not set
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are kept:
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```cpp
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json defaults = {{"log", {{"level", "info"}, {"file", "app.log"}}}, {"retries", 3}};
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json user_settings = {{"log", {{"level", "debug"}}}};
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json config = defaults;
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config.update(user_settings, true);
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// {"log":{"file":"app.log","level":"debug"},"retries":3}
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```
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[JSON Merge Patch](merge_patch.md) ([RFC 7386](https://tools.ietf.org/html/rfc7386)) also merges objects recursively,
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but it is a different tool: a `#!json null` in the patch means "remove this key". It is meant for applying merge patch
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documents (e.g., received via HTTP PATCH). To merge configuration-like objects, use `#!cpp update(..., true)`. To apply
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a sequence of well-defined edit operations, see [JSON Patch](json_patch.md).
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## Removing elements
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Elements are removed with [`erase`](../api/basic_json/erase.md), which accepts an object key, an array index, or an
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iterator. [`clear`](../api/basic_json/clear.md) empties a value while keeping its type, and
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[`operator[]`](element_access/unchecked_access.md) combined with assignment can overwrite a value entirely.
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```cpp
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json j = {{"a", 1}, {"b", 2}, {"c", 3}};
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j.erase("b"); // {"a":1,"c":3}
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json a = {1, 2, 3, 4};
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a.erase(1); // [1,3,4] (erase by index)
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```
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## See also
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- [`push_back`](../api/basic_json/push_back.md) / [`emplace_back`](../api/basic_json/emplace_back.md) - append to an array
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- [`emplace`](../api/basic_json/emplace.md) - insert into an object if the key is absent
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- [`update`](../api/basic_json/update.md) - merge objects (shallow, or recursive with `merge_objects`)
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- [`merge_patch`](../api/basic_json/merge_patch.md) - apply an RFC 7386 merge patch
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- [`erase`](../api/basic_json/erase.md) / [`clear`](../api/basic_json/clear.md) - remove elements
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- [JSON Patch and Diff](json_patch.md) and [JSON Merge Patch](merge_patch.md) - structured modifications
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